Conventional power engines, e.g., compressor, vacuum pump, fluid rotary machine, employ a various types of drive devices: a swing piston type drive device in which a fluid is repeatedly sucked and discharged by reciprocation of a swing piston connected to a crank shaft; a scroll type drive device in which a fluid is repeatedly sucked and discharged by rotating a rotary scroll with respect to a fixed scroll; a rotary type drive device in which a fluid is repeatedly sucked and discharged by rotating rollers (see Patent Document 1); a screw type drive device; and a vane type drive device.
Especially, in a two-piston/four-head fluid pump, a length of a first arm, which connects a crank pin to a shaft, and a length of a second arm, which connects a piston to a crank pin, are equalized so as to reduce a length of a crank arm by half and double a stroke of the piston (See Patent Document 2).